In that field, and also in a more general way, the beam carrying the information is monochromatic at a frequency identically adjusted in the emitting and the receiving lasers. It is transmitted between them through an optical fiber.
Different types of semi-conductor lasers are known that have very small dimensions and serve in particular for emitting and receiving information data carried from the emitter to the receiver through optical fibers. One may distinguish vertical and horizontal lasers. In both cases, considering for example the emitting laser, modern solutions consist in ensuring the stimulated emission of light in a wave guide made from an active semi-conductor material and in causing modulation by applying to this wave guide an electrical current which is placed under the exciting control of a voltage signal carrying binary data to be transmitted so as to consequently vary the emissivity of the active material or its refractive index.
As concerns the prior art relating to the present invention, one may refer in particular to French patent document FR-A-2 639 773, which recalls the principle of semi-conductor lasers with distributed feed-back, known as DFB, incorporating a Bragg reflector (DBR). An emitting diode laser of this type, implemented as a monolithic structure, comprises superposed layers defining a light guide limited by confinement layers, which itself is made of a ribbon of active electro-optical material coupled with a passive layer. Along a direction of light propagation assumed horizontal, one distinguishes mainly a section where amplification of a stimulated photonic emission occurs at a wavelength which depends upon the material used, a phase matching section, and a section with a periodic Bragg grating the periodicity of which is in relation with a diffraction grating inscribed in the adjacent passive film so as to induce a thickness variation in the wave guide.
The light beam is emitted following the liberation of photons under the effect of an excitation current injected in the first section of the laser and of the selective amplification of the so-called Bragg wavelength caused by the resonance between the Bragg grating and an opposite reflective mirror. A modulation of this wavelength in the light leaving the laser is obtained by controlling the effective refractive index of the active material, using an electrical current applied through the grating section.
Document EP 0395 315 describes a laser of a different type, belonging to the class of the so-called vertical lasers. In this case, the Bragg stratification of the semi-conductor material extends no longer in the longitudinal direction of the laser, but throughout superposed layers parallel to the underlying substrate supporting the assembly, so that the emission of light occurs from the laser surface orthogonal to the longitudinal direction, whereas in horizontal lasers the beam exits through the laser's edge. The same document describes also with ample detail the way how the useful properties are obtained locally in each functional zone from a single semi-conducting crystal, through generating properties differing in the sign of the free carriers and in doping.
In practice, relating to the field of communication by data transmission through optical fibers, the research efforts have mainly been directed, these last few years, towards reducing the size of emitting and receiving lasers, and towards improving the quality by increasing the accuracy of the carrier frequency and the amplitude gain brought by the Bragg diffraction gratings limiting the resonating cavity in monolithic structures of transparent materials. The development prospects for applications of such structures are widely connected to the acquired capability for implementing the needed successive layers by techniques such as molecular beam epitaxy and chemical vapor deposition.